Theoretical Study of the CH₃NO₂ Unimolecular Decomposition Potential Energy Surface

Abstract

The complex potential energy surface for the unimolecular isomerization and dissociation of nitromethane (CH3NO2), including 10 CH3NO2 isomers, 46 interconversion transition states, and 16 major dissociation products, is probed theoretically at the G2MP2//B3LYP/6-311++G(2d,2p) level of theory. The geometries and relative energies for various stationary points are determined and are in good agreement with the available experimental values. Based on the calculated G2MP2 potential energy surface, the possible nitromethane unimolecular decomposition mechanism is discussed. It is shown that the most feasible decomposition channels for CH3NO2 are those that lead to 2CH3 + 2NO2, 2CH3O + 2NO, H2CO + HNO, and HCNO + H2O, respectively. Among them, 2CH3 and 2NO2 are produced by the direct C−N bond rupture of nitromethane, while the formation of the latter three products is initiated by CH3NO2 rearranging first to methyl nitrite or to aci-nitromethane. The C−N bond dissociation energy for nitromethane is calculated to be 61.9 kcal/mol, lower than the nitromethane → methyl nitrite and nitromethane → aci-nitromethane isomerization barriers by 2.7 and 2.1 kcal/mol, respectively. Our results suggest that the CH3NO2 isomerization pathways are kinetically disfavored in view of the relatively high activation barriers, in excess of 60 kcal/mol. The nitromethane decomposition occurs either via the C−N bond rupture or via concerted molecular eliminations.

Isomers and isomerization reactions of four nitro derivatives of methane

“…Second, we gave a comparison of the most concerned relative energies (RE, relative to NM) of interested species. The data in Table 1 show us that our RE results are comparable with those derived from other high-level calculations: (1) for CH 3 1 NO 2 , our B3LYP/6-31111G(3df,2p) result, 59 kcal/mol, is in excellent agreement of the UCCSD(T)/cc-pVQZ (or CBS)//UB3LYP/6-3111G(3df,2p) and MRCISD(10,10)/(aug)-cc-pVTZ//CASSCF(10,10)/cc-pVDZ results of Zhu and Lin 3 and Nguen et al, 10 58.6 (59.8) and 59.2 kcal/mol, respectively, not more than 1 kcal/mol; 64 and 63 kcal/ mol of RE resulted from our and Nguen et al's 10 CCSD(T)/ccpVTZ calculation agrees well with each other; (2) for trans-CH 3 ONO, it seems our CCSD(T) result is better than our B3LYP result for it is closer to high-level MO calculation results; (3) for trans-CH 2 NOOH, our results, 19 and 22 kcal/mol, are very close to G2MP2 result, 21.4 kcal/mol 15 ; (4) and (5) for two TSs of CH 3 NO 2 to trans-CH 3 ONO and of CH 3 NO 2 to trans-CH 2 NOOH, our results are in accord with other results; (6) considering the RE of all species shown in Table 1, the B3LYP and all high-level MO methods give a same increasing order of RE: CH 3 NO 2 , CH 3 ONO, CH 2 NOOH, CH 3 1 NO 2 , TS (CH 3 NO 2 -trans-CH 2 NOOH), TS(CH 3 NO 2 -trans-CH 3 ONO). It shows that both geometry structures and energies predicted by B3LYP/6-31111G(3df,2p) are comparable with those predicted by the high-level methods.…”

“…14 A number of calculations have been carried out at different levels on the structures and energies of isomerization and decomposition reactions of NM. 3,11,12,[14][15][16][17][18][19] The decomposition mechanism of NM proposed according to different theoretical calculations and experiments is usually variable. Semi-empirical MINDO/3 calculations indicate that the isomerization to CH 3 ONO is a favorable pathway for the thermolysis of NM and that the isomerization reaction proceeds via a tight transition state, reaction (2), with a barrier lower than the dissociation energy of direct CÀ ÀN rupture mechanism (reaction 1).…”

Modeling Defect-Induced Phenomena

“…Recent experiments enriched possible scenarios by adding bimolecular [17,18] and ionic [19] decompositions. Theoretical studies propose that nitro-to-nitrite rearrangement and HONO elimination reactions are the most favorable and are about 15 kcal mol −1 lower in energy than C-NO 2 break [20], in some qualitative agreement with recent calculations [21,22]. This is in sharp contrast, both quantitatively and qualitatively, to the earlier ab initio results [23] in which the rearrangement barrier of 73.5 kcal mol −1 was found to be 16.1 kcal mol −1 higher than the C-N bond asymptote, concluding that concerted rearrangements on CH 3 NO 2 potential surface will not be observed, and to recent calculations [24].…”

“…While discrepancies among theoretical efforts are mainly defined by differences in methodologies used, the persistent contradiction between theory and experiment is, in our opinion, due to two main reasons. The first is the complex profile of the potential energy surface of nitromethane [26], which is evidenced by numerous transition states found [22], and represents, most likely, a common feature of many EMs [27,28]. The second is that even excessively exhaustive computational studies on the decomposition of isolated molecules are unlikely to provide a consistent interpretation of experimental measurements of solid-state processes because decomposition in the condensed phase fundamentally differs from that in gas and/or dilute solutions due to catalysis and intermolecular interactions [1].…”

FT-IR spectroscopic investigation of the surface reaction of CH4 with NO x species adsorbed on Pd/WO3–ZrO2 catalyst

“…However, this compound is not observed on acidic oxides such as H-ZSM5 [57]. The theoretical calculations on the homogeneous isomerization and dissociation of nitromethane show that both reaction pathways passing through an initial nitromethane -methyl nitrite and nitromethane -aci-nitromethane rearrangements have very close activation barriers differing by 0.6 [63] or 3 kcal/mol [64]. This leads to the conclusion that both channels are possible and competitive [64].…”